As is known, the electric spindles of numerical control machines are composed of a tool-holder spindle and an electric motor that are housed inside a same outer structural casing, aligned one after the other along the longitudinal axis of the electric spindle, and are mechanically coupled to each other so that the electric motor can drive the tool-holder spindle in rotation about the longitudinal axis. Furthermore, the outer structural casing is specifically structured for being fixed to the spindle-holder carriage or slide of the numerical control machine.
More in detail, the electric motor is usually composed of a substantially cylindrical tubular shaped stator assembly which is rigidly fastened inside a specially provided cylindrical cavity in the casing of the electric spindle, so as to be coaxial to the longitudinal axis of the electric spindle; of a drive shaft that extends coaxial to the longitudinal axis of the electric spindle through the pass-through cavity of the stator assembly and the entire casing of the electric spindle; and of a substantially cylindrical tubular shaped rotor assembly which is rigidly fitted onto the drive shaft inside the stator assembly, so as to be perfectly coaxial to the latter.
The tool-holder spindle in turn is basically composed of a rotating tool-holder hub which is located at the end of the drive shaft of the electric motor, so as to protrude outside the casing of the electric spindle while remaining coaxial to the longitudinal axis of the electric spindle, and is centrally provided with a truncated-cone shank-holder seat which is structured so as to accommodate the shank of a generic tool for numerical control milling machines, while always maintaining the tool perfectly coaxial to the longitudinal axis of the electric spindle; and of a hydraulically or pneumatically operated locking member, which is structured so as to engage and retain, in a rigid and stable though easily releasable manner, the shank of the tool within the shank-holder seat of the hub, so that the drive shaft of the electric motor can drive the tool in rotation about the longitudinal axis of the electric spindle.
More in detail, the tool-holder hub is usually made in one piece with the drive shaft of the electric motor and engages, in a pass-through and axially rotatable manner, a substantially bell-shaped front cap, which is fastened to the casing of the electric spindle to close the front end of the cylindrical cavity that houses the stator assembly and the rotor assembly of the electric motor; while the drive shaft is supported in an axially rotatable manner by two sets of support rolling bearings which are located at the two axial ends of the shaft, one inside the front cap and the other abutting on the rear end of the cylindrical cavity of the electric spindle.
Although working very well, this type of electric spindle has limited operability, because the maximum rotational speed achievable by the electric spindle is closely conditioned by the structure and overall dimensions of the tool-holder spindle, which in turn are closely connected to the type of tool that the tool-holder spindle is designed to use.
Obviously, this operational constraint and the fact that the tool-holder hub is formed in one piece directly on the end of the drive shaft, compel the manufacturers of electric spindles to dimension the entire electric motor of the electric spindle so that its maximum performance is substantially equal to that permitted by the tool-holder spindle.
It is evident that these performance limits create major problems when the same numerical control milling machine must be used for mechanical machining that contemplates using, in rapid succession and on the same workpiece, two different types of tool that respectively require high driving torque and low rotational speeds (roughing tools typically require rotational speeds below 20,000 rpm), or low driving torque and high rotational speeds (finishing tools typically require rotational speeds above 20,000 rpm and usually below 50,000 rpm).
In this case, as electric spindles capable of using both types of tool cannot physically be made, when changing from a high-speed tool to a low-speed tool or vice versa, it is necessary to replace the entire electric spindle block mounted on the milling machine en masse, with all of the problems that this entails in terms of machining times and costs.
To at least partially obviate this problem, disassemblable electric spindles have been recently developed, in which the outer structural casing houses just the electric motor and is structured so as to be stably anchored to the numerical control machine. The outer casing and the electric motor housed therein are moreover structured so as to indifferently connect to two different and distinct tool-holder spindles, each of which is structured to be coupled to the electric motor, in a rigid and stable though easily releasable manner, so as to be driven in rotation by the motor with the correct rotational speed.
Even if the tool-change times of the numerical control milling machine are drastically reduced, tool-holder spindles of the disassemblable type have resulted in a significant increase on the overall length of the electric spindle and, even more important, have significantly complicated the structure of the electric spindle.
Each disassemblable tool-holder spindle, in fact, must be provided with its own hydraulically or pneumatically operated locking member that, obviously, must be adequately lubricated and, even more important, must be controllable by the numerical control machine. Requirements that impose to arrange, in the coupling zone between the electric motor and the tool-holder spindle, a considerable number of hydraulic and/or pneumatic connectors necessary for connecting the tool-holder spindle to the outer casing of the electric spindle that, in turn, is connected to the rest of the numerical control machine.
Despite the drastic reduction in weight with respect to monobloc electric spindles, even the disassemblable tool-holder spindles still have an overall weight of many tens of kilograms, with all of the drawbacks that this entails during the spindle change.